Fatigue resistant pipe string component

ABSTRACT

A pipe string component that connects an offshore platform with a sea bed is provided. The pipe string component includes a threaded tubular connection. The threaded tubular connection includes a male tubular element including a tapered male threading, and a female tubular element including a tapered female threading that cooperates with the male threading by makeup to produce a rigid mutual connection of the tubular elements with radial interference between radial load transfer zones of the threadings. The male and female threadings each have a load flank extending substantially perpendicularly to an axis of the male and female threadings. The radial load transfer zones are at a radial distance from envelopes of thread roots of the male and female threadings. The radial load transfer zones of the threadings comprise at least one surface substantially parallel to an axis of the connection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims the benefitof priority under 35 U.S.C. §120 from U.S. Ser. No. 10/581,360, filedJun. 2, 2006 now U.S. Pat. No. 7,845,687, which is a National Stageapplication of PCT/EP04/13743, filed Dec. 2, 2004 and claims benefit ofpriority under 35 U.S.C. §119 from French Application No. 0314527, filedDec. 11, 2003.

FIELD OF THE INVENTION

The invention relates to a method for improving the fatigue resistanceof a threaded tubular connection subjected to stress variations, saidconnection comprising a male tubular element including a tapered malethreading, and a female tubular element including a tapered femalethreading which cooperates with the male threading by screwing toproduce a rigid mutual connection of said tubular elements with radialinterference between radial load transfer zones of said threadings.

That type of threaded connection is primarily intended for theproduction pipe strings for hydrocarbon or the like wells.

Said radial interference is primarily intended to prevent breakout ofthe threaded connections in service which would be catastrophic, and italso renders the threaded connection far more monolithic.

DISCUSSION OF THE BACKGROUND

Threaded connections of that type are known in which radial interferenceis obtained by contact between thread crest and corresponding threadroot, in particular between the crest of the female thread and the rootof the male thread.

Such contact zones between corresponding thread crests and roots thenconstitute radial load transfer zones for the threadings.

It has been established that, when such a threaded connection issubjected to stress variations, in addition to cracking by fatigue instress concentration zones, for example at the foot of the load flank,micro-cracks appear in contact zones at the thread root, which tend todevelop if high and variable tensile stresses exist in that zone,compromising the fatigue resistance of the connection.

Such phenomena primarily occur in rotary drillpipe strings and haverequired for such products threadings cut in very thick attachedelements termed “tool joints” comprising triangular threads of greatdepth with rounded crests and roots. There is no contact between thosethread roots and crests, nor in general any radial interference. Even ifsuch interference were implemented, the radial loads would betransferred to the thread flanks where the tensile stresses are muchlower than at the thread root. The load flanks which, it will berecalled, are the flanks directed towards the side opposite to the freeend of the tubular element under consideration, make an angle of 60°with respect to the axis of the threaded connection. The stabbing flanksare disposed symmetrically, making the same angle with the axis.

These phenomena also occur in pipe strings connecting an offshoreplatform with the sea bed, under the action of waves, wind, tides andsea currents, which induce variable tensile or bending loads on thestring.

SUMMARY OF THE INVENTION

However, with that type of connection, it is not always possible toproduce threads with a large thread depth and triangular threads run therisk of disengaging or jumping out from the tubular elements in servicein the well.

The invention aims to overcome these disadvantages.

The invention aims in particular at a method of the type defined in theintroduction and provides that the threadings each have a load flankextending substantially perpendicular to the axis of the threadings, andprovides that said radial load transfer zones are at a radial distancefrom the envelopes of the thread roots of the male and female threadingsand form an angle of less than 40° with the axis of the threadings.

The term “envelope of the thread root” means the tapered surface whichenvelops the thread roots which is furthest from the thread crests.

Due to the radial separation of the radial load transfer zones withrespect to the envelopes of the thread roots, the micro-cracks which canform therein are not affected by the tensile stresses existing in thematerial beyond the thread root envelope and thus do not deleteriouslyaffect the fatigue resistance of the connection.

Optional characteristics of the invention, which may be complementary orsubstitutional, will be given below:

-   -   said radial load transfer zones are constituted by i) the crest        of at least one helical protuberance formed on the thread root        of at least one threading with respect to the envelope of the        thread root and ii) the facing zone located on the thread crest        of the corresponding threading;    -   the protuberance or protuberances is/are disposed on the male        thread root;    -   the crest of the protuberances is convexly domed;    -   the protuberances are connected to the thread root via one or        more concave rounded portions;    -   said protuberances are each constituted by the crest of a        helical rib formed on the thread root of the threading under        consideration;    -   said radial load transfer zones comprise the crests of at least        two helical ribs which are in axial succession along the thread        root of the male threading;    -   said radial load transfer zones comprise the crest of a boss        extending from the foot of the load flank to the foot of the        stabbing flank on the thread root of the threading under        consideration;    -   said radial load transfer zones comprise the crest of a boss        bearing on one of the flanks of the threading under        consideration;    -   said facing zones located on the thread crest of the        corresponding threading each have a recessed helix partially        enveloping each protuberance;    -   said radial load transfer zones are constituted by respective        intermediate regions of the stabbing flanks of the male and        female threadings, said intermediate regions forming a smaller        angle with the axis of the threadings than the neighbouring        regions of said flanks;    -   the angle between said intermediate regions and the axis of the        threadings is substantially zero;    -   said radial load transfer zones are ramps constituting the        stabbing flanks of the male and female threadings over the major        portion of the radial height thereof;    -   the angle between said ramps and the axis of the threadings is        in the range 20° to 40°;    -   the angle between said ramps and the axis of the threadings is        about 27°;    -   the invention is implemented in a zone of full height threads        termed perfect threads;    -   the invention is implemented both in a zone of perfect threads        and in a zone of imperfect threads, in particular in a zone of        run-out threads;    -   the profile of the male threading comprises a first concave        rounded portion defining the thread root and tangential to said        ramp;    -   the profile of the male threading comprises a second concave        rounded portion with a smaller radius of curvature than the        first rounded portion and tangential thereto and to the load        flank;    -   a groove defining the female thread root extends axially from a        first wall constituted by the load flank to a second wall which        is connected to the ramp of the female threading;    -   the profile of said groove comprises a central concave rounded        portion framed by first and second rounded concave portions        respectively tangential to said first and second walls and with        a smaller radius of curvature than the central rounded portion;    -   the profile of the female threading comprises a convex rounded        portion tangential to a second rounded portion and to said ramp,        the zone of inflexion between the convex rounded portion and the        second rounded portion constituting the second wall.

The invention also relates to a threaded tubular connection forimplementing the above-defined method, comprising a male tubular elementincluding a tapered male threading, and a female tubular elementincluding a tapered female threading which cooperates with the malethreading by screwing to produce a rigid mutual connection of saidtubular elements with radial interference between radial load transferzones of said threadings.

The threaded connection comprises in accordance with the invention atleast one of the following particularities:

-   -   said radial load transfer zones are constituted by i) the crest        of at least one helical protuberance formed on the thread root        of at least one threading with respect to the envelope of the        thread root and ii) the facing zone located on the thread crest        of the corresponding threading;    -   said radial load transfer zones comprise the crest of a boss        extending from the foot of the load flank to the foot of the        stabbing flank on the thread root of the threading under        consideration;    -   said radial load transfer zones comprise the crest of a boss        bearing on one of the flanks of the threading under        consideration;    -   said radial load transfer zones are constituted by respective        intermediate regions of the stabbing flanks of the male and        female threadings, said intermediate regions forming a smaller        angle with the axis of the threadings than the neighbouring        regions of said flanks;    -   said radial load transfer zones are ramps constituting the        stabbing flanks of the male and female threadings over the major        portion of the radial height thereof, and the profile of the        male threading comprises a first concave rounded portion        defining the thread root and tangential to said ramp;    -   said radial load transfer zones are ramps constituting the        stabbing flanks of the male and female threadings over the major        portion of the radial height thereof, and a groove defining the        female thread root extends axially from a first wall constituted        by the load flank to a second wall which is connected to the        ramp of the female threading.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics and advantages of the invention will now bedescribed in more detail in the following description made withreference to the accompanying drawings.

FIGS. 1 to 6 are partial views in axial cross section of the threadingsof different tubular connections of the invention.

FIG. 7 shows an application of the threads of FIG. 1 on a male tubularelement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The threaded tubular connection shown in part in FIG. 1 comprises a maletubular element 1 and a female tubular element 2 respectively includinga tapered male threading 3 and a tapered female threading 4. The femalethreading 4 has a conventional trapezoidal profile, defining a loadflank 5 which extends substantially perpendicular to the axis of thethreadings, i.e. vertically in the figure, the axis being horizontal, astabbing flank 6 forming a different angle which is, however, close to90° with the axis of the threadings, a thread root 7 and a thread crest8 substantially parallel to the axis, the root 7 and crest 8 beingconnected to flanks 5 and 6 via rounded portions. The direction of theinclination of the flank 6 is such that the helical groove formed by thefemale threading shrinks in the direction of the root 7.

The profile of the female threads 4 can in particular correspond to aprofile designated in the American Petroleum Institute's specificationAPI 5CT as a “buttress” profile.

The “buttress” threading has a taper of 6.25% ( 1/16), 5 threads perinch of length, a load flank angle of +3° and a stabbing flank angle of+10°.

Other threadings, in particular derived from the “buttress” threadingtype, can be used.

The male threading 3 has a load flank 10, a stabbing flank 11 and athread crest 12 located facing flanks 5 and 6 and the thread root 7respectively and orientated in the same manner thereas, as well as athread root 13 located facing the thread crest 8 and which extendsparallel to the axis but which is interrupted by two helical ribs 14,the height of which with respect to the thread root 13 is advantageouslyin the range about 0.2 to 0.4 mm. The crest 12 and root 13 are connectedto flanks 10 and 11 via rounded portions. The two ribs 14 with identicalprofiles and the same pitch as threadings 3 and 4 are offset withrespect to each other in the axial direction to leave a fraction of flatbottom 13 between them, and two other fractions either side of the ribs.The ribs 14 have a rounded crest 15 defining a helical contact linebetween the rib and the female thread crest 8. They are also connectedto the bottom of the male thread 13 via rounded portions.

Because of the disposition of the invention, when threadings 3 and 4 aremade up one into the other so that load flanks 5, 10 bear on each otherand a radial interference fit is obtained between the elements 1 and 2,the radial loads transferred between elements 1 and 2 are transferredvia the contact lines 15 which are at a radial distance from the threadroot 13, so that microcracks which may form there because of stressvariations or slight relative movements cannot develop, the tensilestresses only existing beneath the threading roots inside the envelope Eof the thread root 13 (i.e. below this envelope in FIG. 1).

It should be noted that after makeup, a radial clearance subsistsbetween the crest of the male thread 12 and the root of the femalethread 7. An axial clearance also subsists between the stabbing flanks6, 11, which axial clearance should advantageously be minimized. Theradial clearance between the male thread crest 12 and the female threadroot 7 is in particular a function of the rounded portion between thisthread root and the female load flank 5. The radius of curvature of thisrounded portion should be maximized to limit stress concentrations whichare deleterious to the fatigue resistance. This is the same for therounded portion between the male load flank and the male thread root 13.

FIG. 2 shows part of a male tubular element 1 a and a female tubularelement 2 a provided with respective threadings 3 a and 4 a. Referencenumerals 5, 7, 8, 10 and 12 designate elements that were alreadydescribed above with reference to FIG. 1 and will not be describedagain. In contrast to FIG. 1, the male thread root 13 a extendscontinuously parallel to the axis of the threadings 80 facing the femalethread crest 8. The stabbing flank of the male threading is in threeportions, namely a portion 20 having substantially the same inclinationas flanks 6 and 11 of FIG. 1 and connecting via a rounded portion toroot 13 a, a portion 21 with the same inclination as portion 20,connecting via a rounded portion to the thread crest 12, and anintermediate portion 22 extending parallel to the axis 80 and connectingto portions 20 and 21 via rounded portions. Similarly, the stabbingflank of the female threading comprises three portions, namely portions24 and 25 with the same inclination as portions 20 and 21, locatedrespectively facing them and connected via rounded portions to thethread crest 8 and to the thread root 7 respectively, and an axiallyextending intermediate portion 26 facing the portion 22 and connected toportions 24 and 25 via rounded portions. When the threadings 3 a and 4 aare made up one into the other to obtain radial interference, the radialloads are transferred via portions 22 and 26 of the stabbing flanks,which are radially distanced 84 from the thread root 13 a of the malethreading and the envelope E of the male thread root, thus producing theeffect described with reference to FIG. 1.

The above observations concerning the radial clearance 82 between themale thread crest 12 and the female thread root 7, and the roundedportions between the load flanks and the thread roots are alsoapplicable to the connection of FIG. 2. There is also an axial clearance86 between portions 21-25 and between portions 20-24 of the stabbingflanks.

FIG. 3 partially shows a male tubular element 1 b and a female tubularelement 2 b provided with respective threadings 3 b, 4 b. As with theembodiments described above, the load flanks 5, 10 of the female andmale threadings extend substantially radially and their thread crests 8,12 extend substantially axially. Regarding the thread roots and stabbingflanks, their profiles are defined by a combination of straight linesand rounded portions which is described below, the values for the radiiof curvature being indicated by way of example for a tubular connectionbelonging to a pipe string with an external diameter of 177.8 to 339.73mm (7″ to 13″⅜).

Opposite to the male load flank 10 perpendicular to the axis of thethreaded connection, the rectilinear axial profile of the male threadcrest 12 connects via a convex rounded portion 30 to the stabbing flankconstituted by a straight line 31 which forms an angle of 27° with theaxis and which moves away from the flank 5 in the direction of the axis.At the opposite end to the crest 12, segment 31 is tangential to aconcave rounded portion 32 with a large radius of curvature, more than 1mm, for example of the order of 1.5 mm, which defines the male threadroot, a further concave rounded portion 33 with a radius of curvature of0.3 mm being tangential to the rounded portion 32 and to the radialrectilinear profile of the load flank 10.

The double rounded portions 32+33 minimize stress concentrations at thefoot of the load flank 10.

Opposite to the load flank 5, the axial rectilinear profile of thefemale thread crest 8 connects via a large radius of curvature convexrounded portion 35 to the stabbing flank constituted by a straightsegment 36 with the same inclination as the segment 31. Opposite to therounded portion 35, the segment 36 is tangential to a convex roundedportion 37 with a low radius of curvature which is itself tangential toa concave rounded portion 38, also with a low radius of curvature, thecommon tangent of the rounded portions 37 and 38 forming a zone ofinflexion being inclined in the same direction as segments 31 and 36 andforming an angle of 70° with the axis. The rounded portion 38 isfollowed by two other concave rounded portions 39 and 40 the radii ofcurvature of which are more than and less than 1 mm respectively, therounded portion 40 connecting to the load flank 5. The common tangent tothe rounded portions 38 and 39 is orientated axially and defines thefemale thread root.

The set of rounded portions 37, 38, 39, 40 constitutes a kind of groove.The double rounded portions 39-40 minimize the stress concentrations atthe foot of the load flank 5.

The zone of inflexion between the rounded portions 37, 38 constitutesone of the walls of said groove; the other wall is constituted by theload flank 5.

When threadings 3 b and 4 b are made up into each other, in addition toaxial bearing between load flanks 5, 10 and between stabbing flanks 31,36, radial interference is obtained between the stabbing flanks definedby the inclined segments 31 and 36, which are at a radial distance fromthe envelope E of the male thread root, producing the advantagesdescribed with respect to FIG. 1.

The embodiment shown in FIG. 3 has a certain number of advantages:

-   -   a) the pre-stress generated by the threads bearing both on the        load flanks and on the stabbing flanks reduces the geometrical        stress concentration factor at the thread root;    -   b) bearing at the stabbing flanks 31, 36 eases any possible        axial abutment (shown in FIG. 7) under axial compression and        bending loads.    -   c) The angle of 27° with respect to the axis of the stabbing        flanks 31, 36 (i.e. an angle of 63° with respect to the normal        to the axis) can minimize the torque generated by axial bearing        of said flanks with respect to that generated by radial        interference.

An angle for the stabbing flank with respect to the axis of more than40° renders the contribution of axial bearing on the makeup torquepreponderate and prejudicial. That angle is preferably kept below 30°.

Further, too great an angle requires a substantial reduction in thetolerances on the thread width, which is detrimental to production costsfor the threadings. Similarly, a sufficiently small angle produces acertain flexibility in the thread crest, which distributes the load overthe load flank better.

A stabbing flank angle of less than 20° with respect to the axis, incontrast, results in too much axial hindrance in the threads.

Modifications can be made to the embodiments described and shown withoutdeparting from the scope of the invention. Thus, the two ribs 14 in FIG.1 can be replaced by a single rib or by three or more ribs. The crest ofthe ribs, instead of being a point in axial cross section, can have acertain extent in the axial direction, resulting in a contact surfaceand not in contact line with the female thread crest.

In the embodiment shown in FIG. 4, elements 1 c, 2 c, 3 c, 4 c and 8 ccorrespond to elements 1, 2, 3, 4 and 8 of FIG. 1. The ribs 14 arereplaced by a boss 45 which extends between the foot of the male loadflank 10 and the foot of the male stabbing flank 11 and which connectswith the male thread root 13 c.

In the embodiment shown in FIG. 5, elements 1 d, 2 d, 3 d and 4 dcorrespond to elements 1, 2, 3 and 4 of FIG. 1. A boss 55 is connectedon one side to the male load flank 5 and bears against it, and on theother side to the male thread root 13 d.

In the embodiment shown in FIG. 6, elements 3 e, 4 e and 65 correspondto elements 3 d, 4 d and 55 of FIG. 5. A rib 14 e is present on the malethread root 13 e and the female thread crest 8 e has a recessed helixpartially enveloping the rib 14 e after making up the tubular elements 1e, 2 e such that a radial clearance exists between the remainingportions of the female thread crest and the male thread root.

In the embodiment shown in FIG. 2, the intermediate regions 22 and 26 ofthe stabbing flanks are not necessarily orientated axially, but can beslightly inclined with respect to the axis.

In the embodiments shown in FIGS. 1, 2, and 4 to 6, the angle of theload flank can be slightly negative as described, for example, inInternational patent application WO-A-84/04352 or in the VAM TOPthreaded connection sold by the Applicant (catalogue no. 940,publication date July 1994).

The angle of the stabbing flank can be less than 10° or more than 10°.

FIG. 7 shows the application of the invention as shown in FIG. 1 to athreaded connection the male threading 3 of which includes a portionwith perfect threads 43 of full height and similar to those shown inFIG. 1 and a portion of run-out threads 44 of truncated height whichprogressively reduce from the full height at the junction with theportion 43 to zero when the envelope line E of the thread roots reachesthe outer surface of the tube where the male threaded element is formed.

The ribs 14 at the male thread root can advantageously be implanted bothin the perfect thread zone 43 and in the run-out thread zone 44.

The embodiment of FIG. 7 can also be applied to the threadings of FIGS.2 to 6.

The invention can be applied to many types of radially interferingthreads, with a single threaded portion or with a plurality of axiallydistinct threaded portions disposed on the same tapered surface or on aplurality of radially distinct tapered surfaces.

The taper of the threadings can vary widely, for example between 5% and20%.

The invention claimed is:
 1. A pipe string component that connects anoffshore platform with a sea bed, said pipe string component comprising:a threaded tubular connection, said threaded tubular connectionincluding a male tubular element including a tapered male threading, anda female tubular element including a tapered female threading thatcooperates with the male threading by makeup to produce a rigid mutualconnection of the tubular elements, wherein: the male and femalethreadings each have a load flank extending substantiallyperpendicularly to a longitudinal axis of the male and femalethreadings, the male threading comprises a male stabbing flank disposedbetween a male thread crest and a male thread root, the female threadingcomprises a female stabbing flank disposed between a female thread crestand a female thread root, the male stabbing flank and the femalestabbing flank each comprises: an intermediate region; a firstneighboring region adjacent to a first end of the intermediate region;and a second neighboring region adjacent to a second end of theintermediate region, a contacting region between the male and femaleintermediate regions defines a male radial load transfer zone and afemale radial load transfer zone, the male and female radial loadtransfer zones effect a radial interference between the male and femalethreadings upon makeup, the male and female radial load transfer zonesare spaced apart from envelopes of thread roots of the male and femalethreadings by a first radial distance, and the contacting region betweenthe male and female intermediate regions is substantially parallel tothe longitudinal axis of the male and female threadings.
 2. The pipestring component according to claim 1, wherein: the contacting regionbetween the male and female intermediate regions defines a first line, asurface of the first neighboring regions defines a second line, and thefirst line forms a smaller angle with the longitudinal axis of the maleand female threadings than with the second line.
 3. The pipe stringcomponent according to claim 2, wherein the intermediate regions aresubstantially parallel to the longitudinal axis of the male and femalethreadings.
 4. The pipe string component according to claim 2, whereinthe male first neighboring region is spaced apart from a mutually facingportion of the female first neighboring region by an axial clearance. 5.The pipe string component according to claim 1, wherein the male andfemale radial load transfer zones are provided in a zone of full heightthreads.
 6. The pipe string component according to claim 5, wherein themale and female radial load transfer zones are also provided in a zoneof run-out threads.
 7. The pipe string component according to claim 1,wherein the load flanks of the male and female threadings are in contacton at least two consecutive threads.
 8. The pipe string componentaccording to claim 1, wherein at least one stabbing flank of the maleand female threadings comprises a portion defining an angle of about 10°relative to the longitudinal axis of the male and female threadings. 9.The pipe string component according to claim 1, wherein the envelope ofthread roots of one of the male tubular element and the female tubularelement is spaced apart from crests of the other of the male tubularelement and the female tubular element by a second radial distance. 10.A pipe string component that connects an offshore platform with a seabed, said pipe string component comprising: a threaded tubularconnection, said threaded tubular connection including a male tubularelement including a tapered male threading, and a female tubular elementincluding a tapered female threading that cooperates with the malethreading by makeup to produce a rigid mutual connection of the tubularelements, wherein: the male and female threadings each have a load flankextending substantially perpendicularly to a longitudinal axis of themale and female threadings, the male threading comprises a male stabbingflank disposed between a male thread crest and a male thread root, thefemale threading comprises a female stabbing flank disposed between afemale thread crest and a female thread root, a contacting regionbetween the male stabbing flank and the female stabbing flank defines amale radial load transfer zone and a female radial load transfer zone,the male and female radial load transfer zones effect a radialinterference between the male and female threadings upon makeup, themale and female radial load transfer zones are spaced apart fromenvelopes of corresponding thread roots of the male and femalethreadings by a radial distance, and the male and female radial loadtransfer zones of the threadings are substantially parallel to thelongitudinal axis of the male and female threadings.
 11. A pipe stringcomponent that connects an offshore platform with a sea bed, said pipestring component comprising: a threaded tubular connection, saidthreaded tubular connection including a male tubular element including atapered male threading, and a female tubular element including a taperedfemale threading that cooperates with the male threading by makeup toproduce a rigid mutual connection of the tubular elements, wherein: themale and female threadings each have a load flank extendingsubstantially perpendicularly to a longitudinal axis of the male andfemale threadings, the male threading comprises a male stabbing flankdisposed between a male thread crest and a male thread root, the femalethreading comprises a female stabbing flank disposed between a femalethread crest and a female thread root, a contacting region between themale stabbing flank and the female stabbing flank defines a male radialload transfer zone and a female radial load transfer zone, the male andfemale radial load transfer zones effect a radial interference betweenthe male and female threadings upon makeup, the male and female radialload transfer zones are spaced apart from envelopes of thread roots ofthe male and female threadings by a radial distance, stabbing flanks ofthe male and female threadings each include: a first non-contactingregion adjacent to a first end of the contacting region of the male andfemale radial load transfer zones; and a second non-contacting regionadjacent to a second end of the contacting region of the male and femaleradial load transfer zones, and a portion of the male firstnon-contacting region is spaced apart from a mutually facing portion ofthe female first non-contacting region by an axial distance.